Consider a systematic approach for safety monitoring

Any facility dealing with bulk hydrocarbons must monitor
safety conditions at all times. A leak of flammable or toxic
material can occur at any time, and explosion is an
ever-present threat when combustible gases are accidentally
released into working areas of a plant.

Operators have a wide choice of safety equipment to ensure
coverage, backed by a library full of standards and
regulations, but what would be an ideal solution? The
ideal safety instrument would be able to
immediately and accurately sense flammable and toxic gases as
well as flames. It would be reliable, low power and resistant
to poisoning and other environmental effects. Equally
important, it would be low maintenance, economical, flexible,
adaptable and self-protecting. Obviously, a single instrument
such as this just does not exist, but by combining the right
technologies to a particular application conditions, the
ideal level of coverage and performance can be
achieved.

One way to look at protective measures is to rank them into
three categories or stages: Detect > Distinguish > Defend
(Fig. 1). Knowing the technologies used in
each stage, their advantages and their limitations can help facilities achieve optimum
solutions.

Fig. 1 One way to look at
protective measures is to rank them
into three categories or stages: Detect >
Distinguish > Defend.

Three stages of protection

The first line of defense (and the fastest) is a
solution providing wide-area coverage that can immediately
detect the initial symptoms of an issue, under any
environmental conditionby sensing a byproduct of an
actual leak occurring. Ultrasonic gas leak detection can
provide this kind of performance; it instantaneously responds
to the ultrasound generated by a high-pressure gas leak, and it
is practically immune to any environmental conditions. It also
covers a large, spherical radius around the instrument. Yet,
while this technology provides reliable
detection in conditions where traditional solutions cannot, it
reveals nothing about where or what exactly is leaking.

The second line of defense is fixed-point gas detection,
which provides more specific information, such as: What has
been released? Where is the point of the release? Is it a toxic
gas or flammable gas, and at what concentration?

A fixed-point gas detector can provide that information very
quickly, if not quite as fast as the ultrasonic detector, and
will provide operators with an almost exact location of the
release in their facility. One limitation of a traditional
fixed-point gas detector is that it requires the gas to make
contact with the sensing element, or, in the case of
line-of-sight detectors, requires the gas cloud to enter the
detection beam. Outdoor applications can be problematic for
these technologies, as wind and even light airflow can move the
gas away from where it can be detected, even if the leak occurs
right beside a sensor. Temperature and humidity can also
negatively affect the performance of fixed point detection
sensors, and, of course, they all require regular maintenance
and periodic sensor replacements.

Fire or flame detection is the last line of defense to avert
a catastrophe. The fire has started, but hopefully it is still
small, and immediate action must be taken to prevent a
potential disaster. Eliminating the risk of an uncontrollable
fire developing is achieved through the use of automatic
suppression systems, generally combined with a voting system to
ensure that false alarm sources do not trigger these systems.
There are many factors to consider when selecting your flame
detection technology and configuring a system. Eliminating
nuisance false alarm events is of primary concern, as well as
ensuring fast, reliable performance in your particular
environment.

When consulting with providers on equipment choices, it is
beneficial to select an equipment vendor that offers the entire
spectrum of detection solutions. Not only will this simplify
procurement but it makes it possible to have experts available
to consult on the optimum combination of technologies, and
their correct installation, within your applications.

DETECT: ULTRASONIC DETECTORS

An ultrasonic detector is an acoustic device tuned to
frequencies above the range of human hearing (Fig.
2), plus associated amplifiers, signal processors and
alarm circuitry. There are two acoustic sensing types
available: microphone-based and piezo-electric-based sensors. A
single ultrasonic detector can cover a radius of 20 meters (m)
to 40 m, depending on the model required to deal with
background ultrasonic noise sources, as shown in Fig.
3. This technology is almost completely insensitive to
environmental conditions such as precipitation, wind and
humidity. Microphone-based sensors do require periodic maintenance and calibration, while
piezo-electric-based sensors are calibrated for life at the
factory. Piezo-electric sensors have the further advantage of
being non-consumable (never expiring) and use a ceramic seal,
making them extremely robust. Both technologies come with
automatic integrity testing to ensure continuous operation. A
huge advantage with ultrasonic detection is that the response
is essentially instantaneous even under the most extreme
conditions. For added redundancy, some ultrasonic detectors are
available with up to four independent sensors (Fig.
4), as well as features like field-selectable dB alarm
levels and time delays and a choice of either electronic or
true broadband sensor self-testing.

Fig. 2 An ultrasonic detector
consists essentially of an acoustic
device tuned to frequencies above the range of
human hearing.

Fig. 3 A single ultrasonic
detector can cover a radius of 20 m
to 40 m, depending on the model required to deal
with
background ultrasonic noise sources.

Fig. 4 For added
redundancy,
some ultrasonic detectors are
available with up to four
independent sensors, as well as
features like field-selectable dB
alarm levels and time delays to
optimize installation settings.

One limitation, as noted previously, is that an ultrasonic
detector provides no information on what is leaking. In
addition, it is not suitable for low-pressure leaks (below 2
bar/30 psi) and can be activated by other sources of ultrasound
(like pressure relief valves) if not configured correctly.
These devices would not fall into a low-power consumption
category, and the units themselves are slightly larger than
fixed-point detectors and not as flexible to install and
operate (no sensor/transmitter separation).

Manufacturers generally recommend advanced mapping services
to develop the optimum instrument coverage required at a
facility by factoring in potential leak sources and the
complete area of coverage when evaluating background noise to
establish sensor ranges and settings, combining them with
fixed-point solutions.

DISTINGUISH: FIXED-POINT GAS DETECTORS

Gas detectors are available for both combustible and toxic
gases. Combustible gas sensor types include catalytic bead and
infrared (IR), both point and open path, while toxic gas
sensors include electrochemical and metal oxide semiconductor
(MOS).

Combustible gas detectors

A catalytic bead device (Fig. 5) consists
of a pair of catalytic beads with platinum coils inside, one
for sensing and the other, a reference, made the same way but
not catalytically active. Current through the platinum (Pt)
coils warms the beads, and any combustible gas touching the
sensing bead will oxidize, raising the beads temperature
and changing the electrical resistance of the Pt coil inside
linearly with gas concentration. Simple circuitry (i.e.,
Wheatstone bridge) compares the resistances of the sensing and
reference coils to produce an output.

Fig. 5 A catalytic bead sensor
consists of a pair of catalytic beads
with Pt coils inside, one for sensing and the
other, a reference,
made the same way but not catalytically active.
Current through the
Pt coils warms the beads, and any combustible gas
touching the
sensing bead will oxidize, raising the beads
emperature
and
changing the electrical resistance of the Pt coil
inside linearly
with the gas concentration.

Catalytic bead sensors have the advantage in that they detect
both hydrocarbons and non-hydrocarbon combustible gases
simultaneously, which is useful where more than one gas or
non-hydrocarbon gases may be present. A representative sensor
can detect methane, propane, n-butane, isobutylene, hydrogen,
ethane, pentane, hexane, heptane, ethylene, propylene,
methanol, ethanol and more.

This technology has been proven in the field for more than a
half century, and the sensors are simple, rugged and
inexpensive to replace. They are resistant to difficult environmental factors like
condensation and humidity, and, because they have no optics,
are well suited to dusty and dirty environments. They are also
available with custom calibration factors to allow some
flexibility with calibration.

Considerations include susceptibility to some poisoning
agents and the need for an oxygen atmosphere. Exposure to high
concentrations of gas can significantly degrade the
sensors performance; if the exposure continues long
enough, it may cause the bead to overheat and fail prematurely.
Some sensors are self-protecting, with a feature that
automatically reduces current to the bead under these
conditions, dramatically increasing the sensors lifespan.
And compared to IR sensors, catalytic bead sensors do require
more frequent calibration and periodic replacement.

IR gas detectors. These items sense gases
through the principle that hydrocarbon gases attenuate certain
wavelengths of IR light. The particular wavelengths are
specific characteristics of the gases. In analytic work,
technology referred to as nondispersive infrared (NDIR) is used
to identify them. An emitter (usually some form of incandescent
lamp) produces a beam of wideband IR light that is sent through
the air to a wavelength-sensitive sensor. In most designs, the
light from the source is split in two beams, with one going
through the air to be monitored and the other through clean air
as a reference.

IR sensors are available in both point and open-path
versions; the point type uses an IR service life entirely
within the sensor, while the open-path type sends a beam of IR
light through an area to a receiver or to a reflector that
sends the beam back. They are quite accurate (some using
dual-light sources); are immune to poisoning and other
environmental effects. Some units have automatic internal
compensation for changes in temperature, humidity and light
source aging, and others have long calibration intervals.
Unaffected by high concentrations of hydrocarbons, most have
electronic failsafe monitoring, and can function in the absence
of oxygen or in oxygen-enriched atmospheres. They are
economical when considering the service life of the instrument,
with sensor lifetimes of eight to ten years, and typically only
require a simple calibration once a year.

IR sensors are not available for all gases (hydrogen, vinyl
chloride and acetonitrile, for example, cannot be detected by
IR), and are an expensive technology compared to the catalytic
bead. They provide a variable, nonlinear response and are
configured to detect a single target gas, although some units
are available with field selectable combustible gas curves to
adapt to varying plant conditions.

Toxic gas sensors. The most common
technologies used for sensing the presence of toxic gases are
electrochemical and MOS. Electrochemical sensors are
essentially tiny fuel cells. The gas to be detected enters the
sensor through a gas-permeable membrane or a capillary and
comes in contact with an electrolyte with a sensor electrode
and a counter electrode. A reaction (oxidation or reduction)
takes place between the sensor electrode and the gas, and a
current is generated that is linear with gas concentration.
Many sensors add a third (or even a fourth) electrode to
eliminate problems with electrode polarization and to increase
lifetime.

Electrochemical sensors can detect all sorts of gases,
including H2S, CO, NO, NO2,
NH3, SO2, Cl2, H2,
HCN and HCl. Electrochemical sensors are a proven technology,
known to be repeatable, reliable and accurate. Advantages
include specificity of response (only the target gas is
detected, although there can be exceptions), the ability to
detect gases in the parts-per-million (ppm) range, and very
low-power consumption.

Limitations include a tighter temperature range, limited
shelf life and possible cross-sensitivity to other gases, and
that the sensors are not self-protecting. One other factor to
consider is sensor life. Sensors gradually deteriorate and must
be replaced. One year to three years of lifetime is typical,
depending heavily on environmental contaminants, temperature
and humidity. Sensor life tends to be shorter under extremely
hot and dry conditions.

MOS sensors work by detecting a change in electrical
resistance when a target gas like hydrogen sulfide
(H2S) or ammonia (NH3) is adsorbed onto
the surface of a layer of metal oxide. Since response
characteristics change with temperature, a built-in heater
maintains a constant sensor temperature, allowing the sensor to
operate over wide ranges of temperature and humidity.
Advantages of the latest nano-enhanced MOS sensors include the
ability to detect small concentrations of gases (less than 20
ppm), low cost, long life and fast response.

MOS sensor limitations include the power consumption needed
to keep the sensing element hot (about 1 W, in some cases) and
the tendency to be fooled by the presence of non-target gases
like ozone, Cl2 and several others. Sensors
monitoring for particularly dangerous gases like H2S
use special methods to reduce sensitivity to interfering gases.
In addition, some traditional MOS sensors have what most
applications would consider a dangerously slow response and
have a tendency to fall asleep, losing sensitivity
unless exposed to a gas mixture on a regular basis, also known
as a bump test. Like electrochemical sensors they
also require periodic calibration (every three months). MOS
sensors have a 12-minute to 15-minute warmup period and need to
be powered up on location at least 48 hours before the first
calibration. Finally, MOS sensors typically have a moderately
higher cost.

An advancement in MOS technology, in which the surface of
the oxide has nano-scale features, provides improved
performance, including much faster response, no loss of
sensitivity, quick recovery from high-concentration gas
exposure events (a common issue with traditional MOS sensor
materials), and the ability to perform well under hot/dry
conditions. Some also require much less power than conventional
MOS sensors.

Transmitters for use with gas detectors are available in
blind, which generally means no display but with an
LED indication, and full-character display configurations.
There are transmitters available in the market with only
single-channel, accepting a single sensors capabilities,
as well as multi-channel variations that can communicate to
connected sensors independently. Ideally, the selected
transmitter platform would be truly universal and allow all
sensor technologies to be mixed and matched as desired. Other
features to consider when selecting a transmitter are low power
consumption and a wide voltage range, a bright display that can
be read under extreme conditions, a full-character interface
with non-intrusive controls and intuitive commands, plus the
availability of multiple output protocols including
wireless.

DEFEND: FLAME DETECTORS

Flame detectors are sensitive to energy emissions from actual flames, and
are designed to minimize their sensitivity to potential sources
of interference such as sunlight, electric lighting, arc
welding and hot objects. Optical flame detectors utilize
ultraviolet (UV) and IR technologies in various
combinations.

IR detectors work at the spectrum below
(longer wavelength than) visible light, and are available in
near-infrared (NIR), narrowband and wideband types. NIR
detectors are sensitive to wavelengths from 0.7 µm to 1.1
µm and are relatively immune to attenuating sources like
water or vapor. Narrowband IR detectors look for the 4.3
µm emission characteristic of hot CO2 given
off by hydrocarbon fires. Wideband IR detectors
are sensitive to all wavelengths of IR, and can detect some
fires that narrowband units cannot. Note: That water, ice, snow
and steam all absorb IR energy and can affect all but NIR
detectors.

Triple IR detectors (Fig.
6) look for three specific wavelengths: the 4.3
µm of CO2 plus two other wavelengths above and
below that. They respond only if the 4.3 µm emission is
sufficiently greater than the other two, which helps greatly to
reduce false alarms caused by sunlight, welding, lightning,
x-rays, arcs and sparks and by hot objects in the area that are
not on fire. They are also resistant to the effects of known IR
absorbers like rain and fog. Electronics that monitor for the
1-Hz to 20-Hz modulation (flicker frequency) of a
flame can greatly reduce false alarms caused by heat radiation
from equipment, but the detectors still may be sensitive to
very strong, nearby, modulated IR energy sources. In these
cases, the detector will need to be repositioned or can have
shades installed to block an identified false alarm
source.

Fig. 6 Triple IR detectors
look for three specific wavelengths:
the 4.3 µm of CO2 plus two other
wavelengths above and below
that; they respond only if the 4.3 µm
emission is sufficiently
greater than the other two, which helps greatly to
reduce false alarms.

Other advantages of IR detectors include a wide temperature
range, very limited calibration required, and low power
requirements. Low power requirements provide significant cost
savings during installation and the life of the instrument, and
deliver stable performance in applications with dirty power or
periodic power fluctuations. Most flame detectors require an
external reflector to perform automatic visual integrity
testing, while some apply a sapphire window to eliminate this
external reflector, which reduces maintenance and faults
related to a failed self-test from lens fouling. Flame
detectors that come with a wide operating voltage range, a
modular design for easy electronics replacement,
field-selectable sensitivity and time delay, an automated
visual integrity self-test and an available external test lamp
are best suited for the challenges that are presented with
optimizing a detector into its environment.

UV detectors look for the UV light emitted
by a flame. A common operating range is 185 nm to 260 nm, which
eliminates problems with sunlight and incandescent or
fluorescent lighting. A drawback to UV sensors is that
petroleum products tend to be opaque to UV wavelengths, which
means that a UV sensor requires periodic cleaning to remove any
oil that has accumulated on the lens.

Other materials that may affect sensitivity include smoke,
dust, dirt, oil and grease, silicone based cleaners and
standard window glass, all of which absorb UV energy. In
addition, UV sensors are susceptible to false alarms caused by
some types of lighting and by UV sources like arc welders
operating in the vicinity.

UV/IR detectors combine a UV detector with
an IR detector, and respond only if both UV emissions in the 185 nm to 260 nm
range and IR emissions in the 4.4 µm range
are present simultaneously. These ranges are sensitive to
hydrocarbon and metal-based flame, while
special tuning is required for hydrogen and silane fueled
fires. UV/IR detectors offer good false alarm immunity and fast
response, and are suitable for indoor and outdoor applications.
Precautions in their use stem from the fact that some airborne
contaminants absorb UV radiation, and that contaminants on the
detector lens (steam, oil film or smoke) can reduce
sensitivity.

Desirable features

Traditional UV and IR sensors check the integrity of
their optical paths by projecting a beam of UV or IR light
to an external reflector (usually made of metal) that reflects
the beam back to the detector. But this visual integrity test
method has several limitations. The reflector can be degraded
by accumulations of airborne contaminants (it can get dirty) or
be corroded, and require periodic cleaning. It can be knocked
out of alignment, creating a fault condition. The
detectors lens can also get dirty, reducing its
sensitivity; in some cases, the lens can be coated with a
material that blocks flame signals, but allows the test beam to
pass through normally, giving a false impression of integrity.
Fortunately there are IR sensors available with local emitters
that can check sensor function without the use of an external
reflector, which can save a substantial amount of maintenance.

Summary

Hazard detection should be divided into three levels
to provide the most comprehensive coverage. The choice of which
technology to apply depends on
individual applications and the conditions present at the
facility. Multiple detection methods are often the best
approach. When selecting a safety monitoring solution, it is a
good practice to look closely at suppliers with a wide range of
technologies and sensor types, and to work with a single and
experienced source that can provide knowledgeable consultation
on the best approach when considering all the options
available, and ultimately will support the complete solution.
HP

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Well illustrated issuance about "3D" steps to save facilities against dangerous gas release. Such "3D" safety path must be completed with or within a hazop study (L, P, T, C) to define and also install safeguards against such risky problem. Overall, at the case, when such gas release is hydrogen leakage that is not easily detected , even when such gas is burning indeed.

Raymond P. Smith06.25.2014

A well written and very informative overview.

A couple of extra tips:-High integrity ultrasonic gas detection in conjunction with point gas detection offers the prospect of operating traditional overhead deluge systems in modules to mitigate (reduce) the explosion overpressure per published data. High integrity ultrasonic currently being field trialed.

For triple IR flame detectors, (confirmed my manufacturer)utilising the two outer 'non overlapping' bands only means the detector can potentially be used as a stand alone double knock device.

Both the above comments potentially give you less bang for your buck.

T V Venkateswaran06.25.2014

Some details about PID VOC detectors, say for benzene, would have been useful